Researchers from McMaster University in Ontario, Canada, noted this trend continues into adulthood. They argued more should be done to prevent this drop in physical activity.

The researchers interviewed 683 Canadian teens aged 12 to 15. The teens were interviewed twice a year for 12 years until they were 24 to 27 years old.

During that time, the amount of exercise the teens did dropped 24 percent. The biggest decline, the researchers noted, was among young men who were headed to college.

"This is a critical period, as the changes in physical activity during the transition from late adolescence to early adulthood represents the most dramatic declines in physical activity across a person's life," said the study's principal investigator, Matthew Kwan, a postdoctoral fellow in the department of family medicine at the Michael G. DeGroote School of Medicine, in a university news release. "In particular, the transition into post-secondary is a one-time period when individuals become much less active."

The study was published Dec. 15 in the American Journal of Preventative Medicine.

The researchers pointed out that young women did not have the same significant drop in physical activity as the men. Their overall level of physical activity fell only 1.7 percent. High school girls, however, were less active than high school boys to begin with.

"It may be that girls experience the greatest declines in physical activity earlier in their adolescence," Kwan explained.

The study also showed that although other risky behaviors, such as smoking and binge drinking got worse as the teens became young adults, these behaviors declined with age and maturity.